Apparatus and method for detecting leaks in packages

ABSTRACT

This invention detects leaks in small, hermetically sealed packages, especially microchips or other packages of electronic circuits. The invention includes a procedure for detecting fine leaks, and a somewhat different procedure for finding gross leaks. To detect gross leaks, one places the package in a chamber, and varies the pressure in the chamber slightly. If the leak is not too big, one wall of the package, such as its lid, initially becomes deformed, but quickly returns to its original position, due to the leak. If the leak is very large, the wall of the package may not move at all. The position of the wall is monitored with an interferometer, preferably an electronic shearography apparatus. The movements of the wall show whether there is a gross leak. In the fine leak test, the package is placed in the chamber and the pressure is changed substantially, thus causing the walls of the package to deform. If there is a fine leak, a deformed wall gradually returns to its initial position. This gradual return can be measured by the interferometer, and the rate at which the wall returns to its starting position can be used to calculate the leak rate. The interferometer can be located inside the test chamber, or it can be located outside the chamber. At least one additional &#34;control&#34; package is preferably placed inside the chamber, alongside the test package, to verify the accuracy of the test results.

BACKGROUND OF THE INVENTION

This invention relates to the field of testing of packages for leaks,especially the testing of small packages used to house electroniccomponents. The invention can also be used in other applications, suchas in the testing of sealed packages which contain food products ormedical devices, or which contain components to be used in outer space.

Micro-electronic components are mounted within packages that must behermetically sealed to prevent contamination of the fragile innercircuitry and the very fine interconnecting wires or circuit traces. Thecavity inside the micro-electronic package is filled with a dry, inertgas, and typically a metallic lid is brazed to the package to providethe hermetic seal. Leakage of molecules of water or oxygen into thepackage cavity can corrode or oxidize the interconnecting wires to thepoint that the circuit fails. In many cases, the loss of one componentcan lead to the failure of a subsystem or the failure of an entiresystem. The reliability of every electronic control system thereforedepends on the reliability of each hermetic seal of each componentpackage.

In the prior art, it has been known to use "tracer" gases to detect fineleaks in microchips. Typically, one places a batch of electroniccomponents into a chamber, and pressurizes the chamber with a gas suchas helium or krypton-85. Then, one removes the components, and lets themsit for a short time, allowing excess gas to dissipate. Then, the amountof gas still leaking from the package is measured with a massspectrometer or radiation detector. In this method, it is oftennecessary to keep the components within the chamber for several hours,depending on the desired sensitivity of the test and the package volume.

To detect gross leaks, it has been known to place a batch of componentsin a tank filled with liquid fluorocarbons, while an operator watchesfor tiny bubbles streaming out of specific packages.

One disadvantage of both of the above methods is that only discretecomponents can be inspected. Components mounted to completed circuitboards cannot be inspected because of the quantity of the components andbecause the circuit boards themselves release gases.

The present invention provides an optical method of detecting leaks insmall packages. The invention preferably employs electronic shearographyas the method of interferometry, thereby making it possible to obtaintest results in "real time". The same apparatus used to detect grossleaks is also used to detect fine leaks, although the methods aresomewhat different. Also, both tests can be performed on componentsmounted directly on a circuit board.

SUMMARY OF THE INVENTION

The apparatus of the present invention includes a chamber for holdingthe package to be tested, and a means for varying the pressure in thechamber. The apparatus also includes an interferometer for observing thedisplacement of a selected wall, or other surface, of the package beingtested. Preferably, the interferometer is a device for practicingelectronic shearography, and includes a computer and video display forobtaining results in "real time", and for making rapid calculations ofleak rates.

The present invention includes a method for detecting gross leaks andanother method for detecting fine leaks. In the gross leak test, thetest package is placed in the chamber, and an initial interferometricimage of a selected wall of the package is taken. The selected wall isnormally the lid of the package. Then, the pressure in the chamber isvaried slightly, and another interferometric image is taken. If the leakis so big that the package is essentially open to the outside, the wallsof the package will not move at all. The second interferometric imagewill be essentially identical to the first. This result implies thatthere is a very large leak in the package. If the leak is not too great,the package wall will initially move slightly outward or inward(depending on whether the pressure in the chamber was decreased orincreased). The second interferometric image will show that the wall hasmoved. Then, a sequence of further interferometric images is taken. Ifthere is a gross leak, the wall will have returned to its initialposition within a short time. The rate of displacement of the wall canbe calculated, and, with knowledge of the other parameters of thepackage, the rate of movement of the wall can be translated into a leakrate.

In the fine leak test, the package is first placed in the chamber, andthe pressure in the chamber is changed substantially, possibly by asmuch as an order of magnitude, or more. The walls of the containerbecome deformed to a much greater extent than in the gross leak test.Then, a sequence of interferometric images is taken. If there is noleak, the deformed walls stay deformed, and there is no significantchange in the position of the walls during a long period of time. Ifthere is a fine leak, the walls will slowly return to their initialpositions, at a rate which is directly proportional to the leak rate.Comparison of interferometric images taken at known intervals can beused to calculate the leak rate.

In one embodiment, the apparatus for performing the interferometricanalysis is located mainly outside of the test chamber. In anotherembodiment, at least a portion of the interferometric apparatus islocated inside the chamber. Placing the interferometer inside thechamber has the advantage that it is not necessary for the laser beam topass through a transparent cover of the test chamber, but it has thedisadvantage in that a larger chamber is required.

It is also desirable to place a "control" package inside the chamber.The control package is a package which is known not to leak. Observationof the control package insures the accuracy of the test. It is alsopossible to use more than one control package, to insure that the firstcontrol package is not defective.

It is therefore an object of the invention to provide a method andapparatus for detecting leaks in packages, especially small packagesused to house electronic components.

It is another object to provide a method and apparatus as describedabove, which method and apparatus are capable of detecting leaks incomponents mounted on circuit boards.

It is another object to provide a method and apparatus as describedabove, wherein the same apparatus can be used to detect fine leaks andgross leaks.

It is another object to provide distinct methods for detecting fineleaks and gross leaks in packages.

It is another object to provide a method of detecting leaks in amicrochip, the method using electronic shearography to derive areal-time picture of the condition of the seal of the microchip.

It is another object to provide a method and apparatus as describedabove, wherein the results of the leak tests can be easily compared withthe same tests performed on at least one "control" package.

It is another object to provide a method and apparatus for detectingleaks, wherein leaks can be detected reliably and without human error,and wherein the test results can be fully documented by recording ofinterferometric data.

It is another object to improve the efficiency with whichinterferometric data are analyzed, in determining leak rates, bylimiting the analysis to those portions of a fringe pattern whichrepresent points of maximum sensitivity.

Other objects and advantages of the present invention will be apparentto those skilled in the art, from a reading of the following briefdescription of the drawings, the detailed description of the invention,and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of the apparatus used topractice the method of the present invention.

FIG. 2 is a schematic diagram of a second embodiment of the apparatusused to practice the method of the present invention.

FIG. 3 is a diagram showing a typical fringe pattern which is obtainedthrough electronic shearography, the diagram also indicating the regionsof maximum interest on the pattern.

FIG. 4 is a graph showing the variation in an interferometric pattern,during an interval of time, the graph giving information about the fineleak rate of a package.

DETAILED DESCRIPTION OF THE INVENTION

In its most basic form, the method of the present invention comprisesplacing a batch of components into a chamber whose pressure can bevaried, and observing the components with a full-field interferometer.When the pressure in the chamber is increased, the lid of each componentwill deform inward. When the pressure is decreased, the lid of eachcomponent will bulge outward. If there is a leak in the seal of the lid,the internal pressure will eventually become equal to the externalpressure, and the lid will return to its original position. The rate ofmovement of the lid is proportional to the leak rate of the seal. Theoptical interferometer measures the amount of deformation of the lid,and repeated measurements of this deformation yield the rate at whichthe lid returns to its initial condition. Although the leak test isdescribed with respect to the package lid, throughout thisspecification, it is understood that the test could be practiced byobserving some other surface, such as a side wall of the container. Theinvention is not limited to observation of the lid only.

One embodiment of the apparatus for practicing the method of the presentinvention is shown schematically in FIG. 1. Housing 1 and transparentcover 3 define chamber 5. The cover, which may be made of glass or ofanother transparent material, is sealed to the housing by seals 4. Thepressure in the chamber can be reduced or increased by pump 7. Thepackage being tested, which is shown as microchip package 9, is placedwithin the chamber. A second package 11, of known integrity, is placedinside the chamber as a "control". The control package 11 should be apackage that is known not to leak, and it should always be within thefield of view of the camera which records the interferogram.

The behavior of a surface of the package, such as its top lid, isobserved by an interferometric apparatus. The interferometric apparatusof FIG. 1 includes laser 13 and expanding lens 15. The system can alsoinclude filters (not shown) for properly controlling the characteristicsof the light. For example, one can use a depolarizing filter if it isnecessary that the light striking the object be unpolarized. Where themeans of interferometry is the shearography apparatus described in U.S.Pat. No. 4,887,899, it is necessary that the light leaving the object beunpolarized, since the apparatus described in the latter patent includesa birefringent material that separates the light reflected from theobject into two orthogonally-polarized rays. Laser light is polarized,and if the test object is metallic, the light reflected from the objectwill retain its polarization. The result would be a pair of rays ofsubstantially unequal intensity, which would degrade the performance ofthe interferometer.

One can also use other types of filters, such as frosted glass, whichcould be used to assure a more even field of illumination.

Lens 15 causes laser light to illuminate the entire contents of thechamber. The apparatus can also operate without an expanded beam,especially when the object is relatively small, i.e. about 2 mm square.Since the diameter of a laser beam is typically about 1 mm, theunexpanded beam can adequately cover such a small object.

The interferometric apparatus also includes a device for generatinginterferograms which, in the preferred embodiment, are shearograms.Thus, the preferred interferometric apparatus includes a shearographycamera 17 which has an optical element 19, a lens 21, a video camera 23,and a computer 25. The interferometric apparatus can be essentially thesame as, or similar to, the apparatus described in U.S. Pat. No.4,887,899, the disclosure of which is incorporated by reference herein.That is, the interferometer is an apparatus for practicing real-timeelectronic shearography. The apparatus can also be similar to theshearography unit described in U.S. patent application Ser. No.07/528,474, filed May 25, 1990, entitled "Apparatus and Method forPerforming Electronic Shearography", the disclosure of which is alsoincorporated herein.

Although electronic shearography is the preferred interferometrictechnique, the invention is not limited to the use of shearography.Other interferometric techniques can be used.

FIG. 2 shows an alternative embodiment of the apparatus of the presentinvention. In FIG. 2, microchip package 29 and control package 31 aredisposed within chamber 27. Pump 33 varies the pressure in the chamber.Laser 35 directs coherent light, through fiber optic cable 37, to anexpansion lens 39, the expansion lens being located within the chamber.The cable makes it possible for the laser to be far from the test area,thus removing the electromagnetic and thermal noise it could create inthe system, and allowing it to maintain stable operation. Shearographycamera 41 is located within the chamber, and should be capable ofoperating in a vacuum environment. The camera should therefore providefor proper venting of the optical components, to prevent them fromdeforming, and must also provide for proper thermal transfer so that thecamera's electrical components operate stably. The shearography cameracan have the same internal components as camera 17 of FIG. 1. The outputof shearography camera 41 is connected to computer 43. The apparatusalso includes a pressure gauge 45, the output of which is connected tothe computer, and a pressure control line 47, connected from thecomputer to pump 33. Thus, the apparatus of FIG. 2 includes a feedbackloop by which the computer can control the pressure in the chamber. Asimilar pressure gauge and pressure feedback loop can also be providedin the embodiment of FIG. 1.

The principal difference between the embodiments of FIGS. 1 and 2 isthat in FIG. 1, the shearography camera is located outside of thechamber, while in FIG. 2, the camera is inside the chamber. Theadvantage of the embodiment of FIG. 1 is that one can maintain a high orlow pressure inside the test chamber while the camera and laser remainoutside and are unaffected by the changed environment in the chamber.Thus, it is possible to use higher pressures in the chamber than can bewithstood by the camera or the laser. On the other hand, the transparentcover must be very thick, so that it does not deform during the test.For this reason, the cover may produce undesirable optical "noise"distortion. The advantage of having the camera inside the chamber isthat such noise is eliminated. The embodiment of FIG. 2 minimizes thenumber of times that the light must pass through the transparent cover.The main disadvantage of the embodiment of FIG. 2 is that it requires alarge chamber. In FIG. 2, the chamber must not only be large enough toaccommodate the camera, but it must also have enough space so that thecamera is far enough from the package so that it can view the entirepackage or circuit board.

Note that it is also possible to use a fiber optic cable in theembodiment of FIG. 1. That is, the laser in FIG. 1 could also be placeda relatively large distance from the package. However, if a fiber opticcable is used, and the light thus emanates from a point inside thechamber, it would again be necessary to increase the size of thechamber, to enable the beam to expand before reaching the package. Inthe latter case, it would be possible to use higher chamber pressures,as such pressures would not affect the optical fiber.

The apparatus of FIG. 1 or FIG. 2 can be used to test for either grossor fine leaks. The following is a description of the procedure fortesting for gross leaks.

First, the package to be tested is placed in the chamber, and ashearogram (or other kind of interferogram) is taken and recorded. Theinterferometric apparatus can generate an image representing thedeformation of the entire lid of the package although, as will beexplained later, one can restrict the analysis to those positions on thelid which are likely to show the greatest changes.

Then, the pressure in the chamber is increased or reduced slightly, anda second interferogram is taken and recorded. It is generally preferableto reduce the pressure, i.e. to create a partial vacuum in the chamber,but either alternative can be used. The amount by which the pressure isincreased or reduced should be only that amount which is necessary todeform the package lid by a small integral number of wavelengths of thelight used to obtain the interferogram.

Suppose, first, that the package has a hole which is quite large. Thiscondition is equivalent to the situation in which the package has no lidat all. Then the second interferogram will be essentially the same asthe first interferogram, because a change in pressure will not cause anydeformation in the lid. The absence of a change between the first andsecond interferograms can be interpreted as the presence of a very largeleak in the package.

Suppose, next, that there is a gross leak, but that the hole is notnearly as large as in the first case. Then, when the pressure in thechamber is varied slightly, the lid of the package will become deformed.Due to the gross leak, the lid soon returns to its original position asthe pressures inside and outside the package become equal. Theinterferometer generates a sequence of interferograms, and the images ofthe latter sequence are compared with the first or second interferogramto determine the leak rate. Note that, if there is a leak, thesubsequent interferograms will become more nearly identical to the firstinterferogram, and will become less correlated with the secondinterferogram.

In the gross leak test, it is preferable to wait for a short intervalafter taking the second interferogram, before taking the subsequentinterferograms. This interval is typically several seconds, and shouldbe long enough to insure that the lid has deformed sufficiently tocreate at least one interference fringe, but not long enough to allowthe lid to return to its original position. Also, an interval isrecommended because it is preferable to allow the air currents in thechamber, caused by the change in chamber pressure, to settle down into asteady-state condition.

The term "gross leak" is, of course, somewhat arbitrary, but it isusually defined to include packages which leak at a rate of more than10⁻⁴ atm-cc/sec. A "fine leak" can therefore be defined to includepackages which leak at a rate of less than 10⁻⁴ atm-cc/sec. Typically,leaks of less than 10⁻⁸ atm-cc/sec are considered too small to cause aproblem in any system, and many standards define the latter rate as thelimit for an acceptable hermetic seal.

The following is a description of the procedure for testing for fineleaks. The pressure in the chamber is varied substantially, possibly asmuch as an order of magnitude or more. More precisely, the amount ofvariation of pressure, for the fine leak test, should be sufficient todeform the package lid by many wavelengths of the light used to make theinterferogram. While no specific amount of deformation is required, itis preferred that the lid be deformed through hundreds of wavelengths.The greater the amount of deformation, the more sensitive the test.

It is now assumed that the package has passed the gross leak test, andthat the seal on the package is therefore sufficiently tight to causethe lid to become deformed. Because the pressure is varied by a largeamount, the displacement of the lid is many times greater, possiblyhundreds of times greater, than the displacement that occurred duringthe gross leak test. When the lid is first deformed, an interferometricimage is taken. Then, a sequence of subsequent interferometric images istaken, over a period that may extend for several seconds or severalminutes. If there are no leaks whatever, the position of the lid willremain unchanged, and the interferometric images will be essentiallyidentical. If there is a fine leak, the lid will slowly return to itsoriginal position. The rate of movement of the lid can be calculated bycomparing the various interferometric images, and by analyzing the rateof deformation indicated in these images. From the rate of movement ofthe lid, and with knowledge of the parameters of the package, one cancalculate the leak rate.

The fine leak test may not be feasible for all types of packages. Forexample, when the lid of the test package is made of a stiff material,such as a ceramic material, the lid may not be readily deformed. Forlids made of very stiff materials, it may be necessary to rely on thegross leak test, either alone or in combination with some othertechnique, such as the test using krypton or helium, described above.

The present invention can sometimes be used in combination with thehelium or krypton gas test. For very, very fine leaks, the method of thepresent invention is not practical because the rate of movement of thepackage lid, for a very fine leak, would be too small, and the testwould take too long. The krypton or helium test, however, is verysensitive, and can be used to detect the smallest of leaks. The presentinvention, on the other hand, can be used for fine leaks that are notthe smallest, and also for gross leaks, and can detect leaks which rangein size as far as the "no lid" condition. The techniques of the priorart do not permit testing for gross and fine leaks with the sameapparatus.

Determination of the leak rate, in both the gross leak and fine leaktests, depends on three parameters other than the measured rate ofchange of lid deformation. These parameters are 1) the amount of thetest pressure, 2) the stiffness of the lid, and 3) the size of theinternal cavity of the package.

The level of the test pressure is important because it determines theextent to which the lid deforms. The greater the pressure, the greaterthe stress on the lid, and the greater the flow through the suspectedleak. Increasing the pressure increases the sensitivity of the test.

The stiffness of the lid is important because, as mentioned above, thestiffness affects the amount by which the lid deforms under theinfluence of a given pressure differential.

The size of the internal cavity of the test package not only defines thelid area, but also determines the amount of gas trapped inside to deformthe lid.

The above-described parameters are related according to the followingequation which gives the maximum deflection of the lid:

    Y.sub.m =0.0138(PR.sup.4 /ET.sup.3)

where

Y_(m) = amount of deflection of the lid, at the point of maximumdeflection (typically the center of the lid), in inches

P = the difference in pressure between the inside and the outside of thepackage, in psi

R = the smaller side of the rectangular cavity of the test package, ininches

t = thickness of the lid, in inches

E = modulus of elasticity of the lid material

In the above equation, it is assumed that the pressure in the chamber isconstant, i.e. that the chamber is not leaking. Therefore, once thepressure in the chamber has been set, any further change in P is dueonly to the change in internal pressure of the package.

It is also assumed, in this test, that the internal pressure of thepackage is ambient pressure, which is a realistic assumption if thepackage leaks. If the package leaks, the interior would have reachedatmospheric pressure; if the package does not leak, the above equationwould not be strictly true, but the method would give correctqualitative results, since the lids would deform under pressure.

A change in Y_(m) causes a change in the interference pattern producedby the interferometer. The computer which is connected to theshearography camera can count the number of interferometric fringesmoving past a given point, to determine the change in Y_(m). The latterresult can be used in the above equation to determine the leak rate ofthe package.

It is especially advantageous to use electronic shearography as theinterferometric method, because electronic shearography makes itpossible to generate and analyze interference patterns easily bycomputer. Thus, given the specific parameters of the package, thecomputer can directly and automatically calculate the leak rate of thepackage. Electronic shearography has the additional advantage that it isrelatively immune to effects of external sources of noise and vibration.With electronic shearography, a real-time pattern can be observeddirectly by the operator of the system, or the interference patterns canbe interpreted automatically by the computer.

The control package 11 (or 31) can be used to detect leaks in thechamber. The behavior of the control package can be used bothqualitatively and quantitatively. In a qualitative test, one can simplyobserve whether the lid of the control package changes position afterthe pressure in the chamber has been set. If so, the chamber must beleaking, and the test results can be voided. In a quantitative test, onecan apply the above equation for Y_(m) in reverse, i.e. by solving forP, to confirm the assumption that the pressure in the chamber has notchanged.

It is also possible that the chamber may be leaking, but only at a smallrate. By observing the movement of the lid of the control package, andwith knowledge of the parameters of the control package, one canestimate the leak rate of the chamber. This leak rate can then be takeninto account in computing the leak rate of the test package.

The control package can be simply a light metallic diaphragm welded to asolid base material with a cavity, the diaphragm being similar in sizeto the lid of an actual microchip package, and preferably similar insize and shape to the lid of the package being tested. In practice,however, no package is absolutely leakproof. The cavity of the controlpackage is therefore preferably made relatively large so that any smallleaks in the control package will not have a large effect on the liddeformation. The larger the cavity, the more sluggish the deformation ofthe lid, and the less significant is the leak in the control package ascompared with that of the chamber. It is also possible to vent theinterior of the control package to the outside of the chamber, in orderto establish firmly that the interior pressure of the control package isat ambient pressure. FIGS. 1 and 2 show the control package being ventedto the outside.

To insure that the control package is indeed leakproof, one can providetwo or more control packages, all known to be non-leaking, within thesame test chamber. With more than two control packages, it is unlikelythat all such packages will be leaking, and it is virtually certain thatthey will not all leak at the same rate.

In monitoring the differential deformation of each package lid, it ishelpful to concentrate on the area of the lid that is most sensitive tochanges in pressure. A calculation "window" can be placed over thissensitive portion of the image. The window location or locations can begeometrically calculated from the position and size of the package lid,located by the operator, or located automatically by the computer using"machine vision" techniques.

When the interferometric technique is shearography, the most sensitivepoints occur at two places on the lid. FIG. 3 shows a typical patternproduced by electronic shearography. This pattern is the combination oftwo shearograms, each shearogram being an interference pattern formedfrom the superposition of two laterally-displaced images of an object.The pattern of FIG. 3 is therefore electronically generated; the fringesare not produced directly through optical interference, but are insteadthe result of subtracting (or otherwise mathematically comparing) twooptically-generated shearograms. Nevertheless, it turns out that thefringe pattern is directly related to the original image from which thepattern is derived. A more detailed discussion of the combination ofshearograms to form such composite interference patterns is given inU.S. Pat. No. 4,887,899, cited above.

The important point illustrated by FIG. 3 is that the present inventionmakes it possible to select an area of interest, on the package beingtested, to minimize the time required for analysis. In general, theareas of interest are those portions of the package lid which movethrough the greatest numbers of wavelengths of light for a given amountof deformation of the lid. By focusing on these areas, one can detectsmall movements in the lid more readily than by examining other areas.

In the shearographic technique described in U.S. Pat. No. 4,887,899,cited above, the pattern obtained comprises sets of concentric fringes.These sets of concentric fringes are shown in FIG. 3. The center of eachset of fringes comprises a point of interest. These points of interestare the points of maximum sensitivity, described above. The rectanglesdrawn around the points of interest represent the "window" discussedabove.

It turns out that if the amount of shearing is very small, the points ofinterest comprise the points of maximum strain on the lid. As the amountof shearing increases, the locations of the centers of the fringepatterns shift from the points of maximum strain to the points ofmaximum deformation. When the amount of shearing is equal to one-halfthe dimension of the lid, the points of interest coincide with points ofmaximum deformation of the lid. As the amount of shearing is increasedbeyond this level, the points of interest continue to be the points ofmaximum deformation.

Thus, one way to determine the areas of interest is to inspect thepatterns visually. One can obtain a shearogram of the lid, and can focusattention on those regions near the centers of the concentric fringesobtained in the shearogram. However, this visual method has thedisadvantage that it may be necessary to take at least one additionalshearogram; one pattern would be taken to determine the areas ofinterest, and another pattern would then be taken to conduct the actualtest. The mathematical method described below is therefore usually morepractical.

The following is a description of one method by which the computer canuse the shearographic data represented in FIG. 3 to choose the area ofinterest automatically.

When one takes a first shearogram, one forms two laterally displacedinterfering images of the same object. These images are designated asY_(1i) and Y_(2i). The subscript "i" means "initial", and the subscripts"1" and "2" refer to the two laterally-displaced images that are made tointerfere with each other. Similarly, when one takes a secondshearogram, after deforming the object, the resulting images are Y_(1f)and Y_(2f), where the subscript "f" means "final". Each individualshearogram is the superposition of the pair of images, and the compositepattern is obtained by comparing the two shearograms, as explained inU.S. Pat. No. 4,887,899. Thus, the composite pattern, which will becalled Y_(s), can be written as

    Y.sub.s =(Y.sub.1i +Y.sub.2i)-(Y.sub.1f +Y.sub.2f).

By rearranging terms, one obtains:

    Y.sub.s =(Y.sub.1i -Y.sub.1f)-(Y.sub.2f -Y.sub.2i).

But the expressions (Y_(li) - Y_(1f)) and (Y_(2f) - Y_(2i)) are reallythe displacements of the first and second images of the pair. Therefore,we can replace these expressions with equations for the deflection ofthe lid, similar to the equation for Y_(m), shown above, by reference toprinciples of the theory of elasticity. That is, we can write

    Y.sub.s =Y.sub.1 -Y.sub.2

where

Y_(s) = the shearographic deflection,

Y₁ = the deflection of the first image, and

Y₂ = the deflection of the second image.

The lateral displacement between the two images, i.e. the amount ofshearing, is also known as the "shear dimension" and will be calleds_(d). In order to determine the points of interest, one must replacethe terms in the above equation with expressions which represent thedeformation of the lid. Then, one solves the above equation for itsextremum points. The extremum points comprise the locations of the areasof interest. It turns out that these areas coincide with the centers ofthe two patterns of concentric fringe lines which appear in a typicalshearogram.

By analogy to the equation for Y_(m), given previously, for the maximumdeflection of the lid, one can write the following expressions forY_(s), Y₁, and Y₂

    Y.sub.1 =[CP/Et.sup.3 ]x.sup.2 (R-x).sup.2

    Y.sub.2 =[CP/Et.sup.3 ](x+S.sub.d).sup.2 (R-S.sub.d -x).sup.2

    Y.sub.s =[CP/Et][(x+S.sub.d).sup.2 (R-S.sub.d -x).sup.2 -x.sup.2 (R-x).sup.2 ]

where

x = linear position, in inches,

P = pressure differential, in psi,

E = modulus of elasticity,

t = lid thickness (inches),

R = smaller side of rectangular cavity (inches),

C = constant = 0.22, and

S_(d) = shearing dimension, in inches.

In this analysis, we assume that the shearing is performed in the xdirection. Thus, for the first image, x=0 could be the beginning of thelid; for the relatively sheared image, s_(d) would be the beginning ofthe lid.

If one solves Y_(s) for its extremum points, which can easily be done inthe above example, one obtains the center of the locus of pointscomprising each set of fringes. The center of the innermost fringe isthe point of maximum interest. The computer can therefore be programmedto locate this point, without operator intervention. The computer canalso solve for the points at which Y_(s) equals an integral number ofhalf-wavelengths, which determines the location of each set of fringes.

As explained above, the method of the present invention includes storingan initial interferometric image (whether taken before or afterpressurization), obtaining a sequence of further interferometric images,and comparing the latter images with the initial image. There are manyways to perform this comparison. One way is discussed below.

The window over each area of interest is composed of a matrix of pixelsfor each frame f of the real time image. We define the correlationcoefficient for frame f by the following equation:

    C.sub.f =1-Σ|(I.sub.f -I.sub.i)|/(ΣI.sub.f +ΣI.sub.i)

where:

C_(f) = correlation between frame f and frame i, the initial frame;perfect correlation is indicated by a value of 1;

I_(i) = intensity of a particular pixel in the initial image; and

I_(f) = intensity of a corresponding pixel in frame f,

and where Σ indicates a summation over all the pixels in the particularwindow.

When C_(f) is plotted against time (which is equivalent to plottingagainst the frame number), one obtains a curve of "decorrelation". Theterm "decorrelation" is used because when the effects of a leak have notyet become apparent, the second images of the lid will be virtuallyperfectly correlated with the first image. As the pressures equalize,due to the leak, and the lid moves, the correlation between subsequentimages and the first image will decrease. Thus, when a package begins toleak, subsequent frames become "decorrelated" from the initial conditionof near perfect correlation.

FIG. 4 shows a sample graph of the above-defined correlation functionover time for a typical leak test. In the test summarized in FIG. 4, thepressure in the chamber was changed from ambient pressure to 7 inches ofmercury (-3.5 psi), and held for ten seconds. The entire test thenlasted about 20 seconds. The resulting curve of decorrelation isapproximately sinusoidal. The slope of the curve is directly related tothe rate of change of the lid deflection Y_(m) as the package leaks. Thecomputer can calculate a measured leak rate based on the instantaneousslope of the curve, and then, using the specific parameters of thepackage, can calculate the actual leak rate of the package. Thisanalysis can be performed on each package in the field of view at nearreal-time speeds, with each test taking approximately one minute. Eachinspection could include analysis of an entire circuit board or of atray of discrete components. Thus, the method of the present inventionrepresents a vast improvement over the leak testing methods of the priorart.

While the invention has been described with respect to particularembodiments, it is understood that variations can be made. Thearrangement of the apparatus of FIG. 1 or FIG. 2 can be changed. Thenumber of control packages can be varied. Although electronicshearography is preferred, the invention is not limited to the use of aparticular type of interferometry. Moreover, although interferometry isconsidered to be the best method for measuring slight displacements ofthe package lid, other techniques for measuring such displacements couldalso be used. For example, one can use a pulse-echo technique, usingeither pulses of electromagnetic energy or acoustic pulses, to detectaccurately the position of a point on the lid. Thus, the invention cantherefore be modified considerably, and such modifications should bedeemed within the spirit and scope of the following claims.

What is claimed is:
 1. A method of inspecting a package for gross leaks,comprising the steps of:a) placing the package in a chamber, b)recording a first interferometric image of the surface of the package,c) varying the pressure in the chamber by an amount sufficient to deforma surface of the package by a small integral number of wavelengths oflight, d) recording a second interferometric image of the surface of thepackage, e) waiting for an interval of several seconds, f) recording aseries of third interferometric images of the surface observed in step(c), and g) comparing the third images with at least one of the previousimages to determine whether the surface has moved during the interval ofstep (d).
 2. The method of claim 1, wherein the interval of step (e) ischosen to be sufficiently long to allow the surface to move by an amountsufficient to form at least one interference fringe in the thirdinterferometric images.
 3. The method of claim 2, wherein the intervalof step (e) is also chosen to be not so long as to allow the surface toreturn to its initial position.
 4. The method of claim 1, wherein theinterferometric images are obtained by electronic shearography.
 5. Themethod of claim 1, wherein the comparing step is performed with respectto only a portion of the surface of the package, and wherein saidportion is near a region of maximum interferometric response on thesurface.
 6. The method of claim 1, wherein steps (a) through (g) aresimultaneously performed on the package being inspected and on at leastone control package.
 7. The method of claim 1, further comprising thestep of calculating a leak rate based on the amount by which the surfacehas moved during said interval.
 8. A method of inspecting a package forfine leaks, comprising the steps of:a) placing the package in a chamber,b) varying the pressure in the chamber by an amount sufficient to deforma surface of the package by a large number of wavelengths of light, c)recording a first interferometric image of the surface of the package,after completion of step (b), d) recording a series of secondinterferometric images of the surface observed in step (c), and e)comparing the second images with the first image to detect changes indisplacement of the surface while said second interferometric images arebeing recorded.
 9. The method of claim 8, wherein the interferometricimages are obtained by electronic shearography.
 10. The method of claim8, wherein the comparing step is performed with respect to only aportion of the surface of the package, and wherein said portion is neara region of maximum interferometric response on the surface.
 11. Themethod of claim 8, wherein steps (a) through (e) are simultaneouslyperformed on the package being inspected and on at least one controlpackage.
 12. The method of claim 8, further comprising the step ofcalculating a leak rate based on the amount by which the surface movesduring step (e).
 13. A method of testing a microchip for gross leaks,comprising the steps of:a) placing the microchip in a chamber, b)recording a first interferometric image of a surface of the microchip,c) slightly varying the pressure in the chamber, d) recording a secondinterferometric image of a surface of the microchip, e) waiting for aninterval of several seconds, f) recording a series of thirdinterferometric images of the surface observed in step (d), and g)comparing the third images with at least one of the previous images todetermine whether the surface has moved during the interval of step (e).14. The method of claim 13, wherein the interval of step (e) is chosento be sufficiently long to allow the surface to move by an amountsufficient to form at least one interference fringe in the thirdinterferometric images.
 15. The method of claim 13, wherein theinterferometric images are obtained from a means for generating aninterferogram, the interferogram generating means being located outsidethe chamber.
 16. The method of claim 13, wherein there are at least twomicrochips within the chamber, and wherein the method is performedsimultaneously on all microchips within the chamber.
 17. The method ofclaim 13, wherein the interferometric images are obtained from a meansfor generating an interferogram, at least a portion of the interferogramgenerating means being located inside the chamber.
 18. The method ofclaim 17, wherein there are at least two microchips within the chamber,and wherein the method is performed simultaneously on all microchipswithin the chamber.
 19. Apparatus or detecting leaks in a microchip,comprising:a) means defining a chamber, the microchip being tested beinglocated inside the chamber, b) an additional microchip located insidethe chamber, the additional microchip being called the "control", c)means for electronically generating a shearogram, the shearogramgenerating means being positioned to detect movements in surfaces ofboth microchips located within the chamber, d) means for analyzingshearograms taken of the microchips in the chamber, and e) means forvarying the pressure in the chamber.
 20. The apparatus of claim 19,wherein the shearogram generating means is located outside the chamber.21. The apparatus of claim 19, wherein at least a portion of theshearogram generating means is located inside the chamber.
 22. Theapparatus of claim 20, wherein the chamber defining means includes atransparent cover.
 23. The apparatus of claim 19, wherein the controlpackage comprises a metallic diaphragm connected to means defining acavity, the diaphragm being similar in size and shape to a surface ofthe microchip being tested.
 24. The apparatus of claim 23, wherein thecavity is sufficiently large such that small leaks in the cavity do notsubstantially affect the results of the test.
 25. The apparatus of claim23, wherein the cavity is fluidly connected to the outside, such thatthe pressure within the cavity is at ambient pressure.
 26. A method ofinspecting a package for gross leaks, comprising the steps of:a) placingthe package in a chamber, b) varying the pressure in the chamber by anamount sufficient to deform a surface of the package by a small integralnumber of wavelengths of light, c) determining the position of thesurface of the package, d) waiting for an interval of several seconds,and e) repeating step (c), so as to determine whether the surface hasmoved during the interval of step (d).
 27. A method of inspecting apackage for fine leaks, comprising the steps of:a) placing the packagein a chamber, b) varying the pressure in the chamber by an amountsufficient to deform a surface of the package by a large number ofwavelengths of light, c) determining the position of the surface of thepackage, after completion of step (b), and d) performing step (c)repeatedly, so as to locate any movements in the surface.